This application claims priority of German Patent Document DE 100 40 125.2 filed Aug. 17, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for reducing the emission of vaporized hydrocarbons in a fuel supply system, in particular for a motor vehicle.
Regulations governing emission of waste gases will become even stricter in the future, and are increasingly including limitations on emissions of vaporized material from vehicles. One source of emissions of this type is the fuel supply system. To prevent the escape of hydrocarbon-containing fuel vapours from a fuel container or a carberator in motor vehicles, fuel supply systems have a closed design and are provided with an activated carbon filter. (See, for example, German Patent document DE 199 52 092). When an internal combustion engine is not running, fuel vapours escaping from the tank region of the fuel supply system go to the activated carbon filter in which the major part of their hydrocarbon constituents are held back and do not get into the environment. When the internal combustion engine is running, the activated carbon filter is flushed with drawn-in air. The hydrocarbons previously absorbed in the filter are thus returned to the internal combustion engine for combustion.
This technique described makes it possible to prevent most hydrocarbon emissions from the fuel supply system. A persistent problem, however is the release of hydrocarbons (hereinafter also designated as HCs for short) from the partly regenerated activated carbon filter, or in an extreme case from the unregenerated activated carbon filter, when the vehicle is not run for a number of days. To meet future limits, this emission has to be prevented.
It is an object of the invention to provide a method and apparatus for reducing hydrocarbon emissions from the filter for binding the hydrocarbons, when the vehicle is not run for a prolonged period.
This and other objects and advantages are achieved by the catalytic reactor apparatus according to the invention, in which a catalyst for oxidizing hydrocarbons to carbon dioxide and water is installed downstream of the existing filter (in particular an activated carbon filter), so that any hydrocarbons exiting from the activated carbon filter can be oxidized in this manner.
As an oxidation catalyst, use may be made, in particular, of noble metals (e.g. Pt, Pd, Rh, Ir or Au) or transition metals (e.g., Mn or Co) which can be applied to a microporous support, such as Al2O3, SiO2, TiO2 or a zeolite.
The catalytic reactor according to the invention can be produced in various designs. The catalyst can, for example, be provided as a bed comprising pellets, spheres or extrudates or be applied as a coating to a geometric support made of ceramic or metal (The term xe2x80x9cgeometric supportxe2x80x9d is used here to distinguish it from a xe2x80x9cmicroporous supportxe2x80x9d; the latter is often also referred to as a xe2x80x9cwashcoatxe2x80x9d). Any necessary heating of the catalyst can be carried out, for example, electrically.
In a particularly advantageous embodiment of the reactor, the catalyst is applied as a layer to an electrically insulating substrate provided with a heating structure, e.g., an aluminium oxide substrate. The advantage of this design is the ability to heat the catalyst directly. In this embodiment, a particularly compact reactor can be achieved.
In a further embodiment, the substrate comprises a PTC (positive temperature coefficient) ceramic. Application of an electric potential enables a defined temperature to be set without further regulation. An additional heating structure is not necessary in this embodiment.
In a further advantageous embodiment, a material which can store HCs is introduced into the reactor in addition to the oxidation catalyst. A particularly important storage mechanism is adsorption. In this process variant, the hydrocarbon is stored over a particular period of time during which no heating of the reactor is necessary. After a particular loading of the storage material has been reached, the reactor is heated to a temperature above the start temperature of the catalyst. This results in desorption of the stored hydrocarbon and reaction of the hydrocarbon in the presence of the oxidation catalyst to form carbon dioxide and water.
A significant advantage of this embodiment is the reduction in the electrical energy required, since no heating of the reactor is necessary during the HC storage phase.
HC storage material and catalyst can be present in physically separate regions or can be mixed with one another. In the former case, the HC storage material should be located upstream (based on the flow direction of the gas) of the catalyst. Thus, for example, catalyst and HC-storing solid can each be present as particulate materials which can be kept separate from one another or mixed with one another. Likewise, catalyst and HC-storing solid can be mixed with one another in powder form (mixture of solids) and applied to a geometric support.
It is also possible for HC-storing solid and catalyst to be arranged in successive layers on a geometric support.
Catalyst and HC-storing solid can also be applied to different sides of an electrically insulating substrate (e.g., an aluminium oxide substrate), with the catalyst being heated directly by means of a heating structure applied to the substrate. In this embodiment, the reactor can be made particularly compact.
In a further embodiment, the microporous catalyst support can also take on the function of the HC-storing solid.
The present invention enables the HC emissions from the fuel supply system of a motor vehicle to be reduced, thus making a contribution to meeting future waste gas limits.
The reactor of the invention for catalytic oxidation and, if desired, for intermediate storage of HCs can be made very small, so that the apparatus of the invention requires little space.
The invention can be used in any type of vehicle, in particular passenger cars.
The invention can also be used, in particular, in a fuel supply system for a vehicle powered by one or more internal combustion engines or for a vehicle powered by means of fuel cells.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.